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  Java Practices -> Launch thread is just another user thread
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<div class='page-title'>Launch thread is just another user thread</div>

<div class='main-body'>
 
<br>Java programs always begin with a <tt>main</tt> method, called in a <em>user thread</em>.

<P>There are two kinds of threads in Java. They are distinguished soley by how they 
affect program termination:
<ul>
 <li><em>user threads</em>, which prevent a program from terminating.
 <li><em>daemon threads</em>, which <b>don't</b> prevent a program from terminating.
</ul>

A program terminates when there are no more <em>user threads</em> left to execute.

<P>In addition, all user threads are created equal. In particular, the first user thread
created upon launch of a program <em>has no special status</em>, and its termination
does not necessarily imply that the entire program will terminate.

<p><b>Example</b>

<p>The <tt>FlightSimulator</tt> class exists only to launch two other (user) threads.
The thread which launches <tt>FlightSimulator</tt> itself quickly ends, but
the program still continues, since there are two other user threads still
running.
<p>Output from a sample run of <tt>FlightSimulator </tt>:
<p><tt>Running Flight Simulator.</tt>
<br><b><tt>Terminating the original user thread.</tt></b>
<br><tt>Running Charles de Gaulle Airport.</tt>
<br><tt>Charles de Gaulle Has Available Runway: false</tt>
<br><tt>Flight 8875: waiting for runway...</tt>
<br><tt>Charles de Gaulle Has Available Runway: true</tt>
<br><tt>Flight 8875: taking off now...</tt>
<br><tt>Flight 8875: flying now...</tt>
<br><tt>Charles de Gaulle Has Available Runway: false</tt>
<br><tt>Charles de Gaulle Has Available Runway: true</tt>
<br><tt>Charles de Gaulle Has Available Runway: false</tt>
<br><tt>Charles de Gaulle Has Available Runway: true</tt>
<br><tt>Charles de Gaulle Has Available Runway: false</tt>
<br><tt>Charles de Gaulle Has Available Runway: true</tt>
<br><tt>Charles de Gaulle Has Available Runway: false</tt>
<br><tt>Charles de Gaulle Has Available Runway: true</tt>
<br><tt>Charles de Gaulle Has Available Runway: false</tt>
<br><tt>Flight 8875: waiting for runway...</tt>
<br><tt>Charles de Gaulle Has Available Runway: true</tt>
<br><tt>Flight 8875: landing now...</tt>
<br><tt>Charles de Gaulle Has Available Runway: false</tt>
<br><tt>Charles de Gaulle Has Available Runway: true</tt>
<br>
<PRE>

<span class='comment'>/**  Builds and starts threads for Airport and Airplanes. */</span>
<span class='keyword'>public</span> <span class='keyword'>final</span> <span class='keyword'>class</span> FlightSimulator {

  <span class='keyword'>public</span> <span class='keyword'>static</span> <span class='keyword'>void</span> main(String... arguments) {
    System.out.println( <span class='literal'>"Running Flight Simulator."</span> );

    <span class='comment'>//build an airport and start it running
</span>    Airport charlesDeGaulle = <span class='keyword'>new</span> Airport(<span class='literal'>"Charles de Gaulle"</span>);
    Thread airport = <span class='keyword'>new</span> Thread( charlesDeGaulle );
    airport.start();

    <span class='comment'>//build a plane and start it running
</span>    Thread planeOne = <span class='keyword'>new</span> Thread( <span class='keyword'>new</span> Airplane(charlesDeGaulle, <span class='literal'>"Flight 8875"</span>) );
    planeOne.start();

    <span class='comment'>//notice that this user thread now ends, but the program itself does
</span>    <span class='comment'>//NOT end since the threads created above are also user
</span>    <span class='comment'>//threads. All user threads have equal status, and there
</span>    <span class='comment'>//is nothing special about the thread which launches a program.
</span>    System.out.println(<span class='literal'>"Terminating the original user thread."</span>);
  }
} 
</PRE>
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<div class='topic-section'>See Also :</div>
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  <a href='TopicAction7721-2.html?Id=51'>Objects communicating across threads</a> <br>
 
  
  <a href='TopicActionc88a-2.html?Id=86'>Use System.exit with care</a> <br>
 
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